In mobile communication, a radio communication base station apparatus (hereinafter abbreviated as “base station”) transmits control information for reporting a resource allocation result of downlink data and uplink data, to radio communication mobile station apparatuses (hereinafter abbreviated as “mobile stations”). This control information is transmitted to the mobile stations using downlink control channels such as a PDCCH (Physical Downlink Control CHannel). Each PDCCH occupies one or a plurality of consecutive CCE's (Control Channel Elements). The base station generates PDCCH's on a per mobile station basis, allocates CCE's to be occupied to the PDCCH's according to the number of CCE's required for control information, maps the control information on the physical resources associated with the allocated CCE's, and transmits the results.
For example, in order to satisfy the desired received quality, an MCS (Modulation and Coding Scheme) of a low MCS level needs to be set for a mobile station that is located near the cell boundary of poor channel quality. Therefore, the base station transmits a PDCCH that occupies a larger number of CCE's (e.g. eight CCE's). By contrast, even if an MSC of a high MCS level is set for a mobile station that is located near the center of a cell of good channel quality, it is possible to satisfy the desired received quality. Therefore, the base station transmits a PDCCH that occupies a smaller number of CCE's (e.g. one CCE). Here, the number of CCE's occupied by one PDCCH (i.e. CCE occupation number) is referred to as “CCE aggregation size.” For example, when the CCE aggregation sizes of 1, 2, 4 and 8 are used, a mobile station that is located near the cell center tries to receive a PDCCH of the CCE aggregation size of 1, and a mobile station that is located near the cell edge tries to receive a PDCCH of the CCE aggregation size of 8.
Also, a base station allocates a plurality of mobile stations to one subframe and therefore transmits a plurality of PDCCH's at the same time. In this case, the base station transmits control information including CRC bits scrambled by the ID numbers of the destination mobile stations, so that the destination mobile station of each PDCCH can be identified. Further, the mobile stations decode CCE's to which PDCCH's can be arranged, and perform CRC detection after descrambling the CRC bits by their mobile station ID numbers. Thus, mobile stations detect the PDCCH's for those mobile stations by performing blind decoding of a plurality of PDCCH's included in a received signal.
However, when the total number of CCE's is large, the number of times a mobile station performs blind decoding increases. Therefore, in order to reduce the number of times a mobile station performs blind decoding, a method of limiting the CCE's subject to blind decoding on a per mobile station basis, is studied (see Non-Patent Document 1). With this method, a plurality of mobile stations are grouped, and CCE fields to include CCE's subject to blind decoding are limited on a per group basis. For example, when a plurality of mobile stations are grouped into UE groups #1 to #4, among CCE's #0 to #31, four CCE fields of CCE's #0 to #7, CCE's #8 to 15, CCE's #16 to 23, and CCE's #24 to 31, are subject to blind decoding in the UE groups, respectively. By this means, the mobile station of each UE group needs to perform blind decoding of only the CCE field allocated to that mobile station, so that it is possible to reduce the number of times of blind decoding. Here, the CCE field subject to blind decoding by a mobile station is referred to as “search space.”
Also, in order to reduce the number of times a mobile station performs blind decoding, studies are underway on a method of limiting in advance the starting location of CCE's occupied by the PDCCH of each CCE aggregation size (see Non-Patent Document 2). With this method, for example, among CCE's #0 to #31, when the CCE aggregation size is 8, the starting locations of CCE's (eight CCE's in this case) occupied by PDCCH's are limited to CCE #0, CCE #8, CCE #16 and CCE #24. By this means, each mobile station needs to perform blind decoding of PDCCH's of a CCE aggregation size starting from the CCE starting locations, so that it is possible to reduce the number of times of blind decoding.    Non-Patent Document 1: 3GPP RAN WG1 Meeting document, R1-073996, “Search Space definition: Reduced PDCCH blind detection for split PDCCH search space,” Motorola    Non-Patent Document 2: 3GPP RAN WG1 #50bis, R1-074317, “Reducing the decoding complexity of the PDCCH,” Nokia